A variety of discrimination systems, operating in a range of circumstances, use color analysis as a basic tool. An example can be found in agriculture, where a green area, such as a weed or other target plant, is to be detected within an area of another color, such as brown soil, in order to be sprayed. However, plant material on the ground, under natural, outdoor conditions, is not easily detected based on color detection if the detection equipment is a conventional video camera. The true color (hue) and brightness of objects may be altered in the video image, depending on the prevailing light level and on whether the object is in direct sunlight or in dark shade.
Color, as perceived by the human eye, can be described in terms of the three components lightness, hue and saturation as illustrated in FIG. 1.
Lightness L* is the perceptual human response to luminance Y, a CIE (Commission Internationale de L'Eclairage) quantity, defined as the radiant power weighted by a spectral sensitivity function that is characteristic of vision. Roughly speaking, areas appearing brighter that emit more light, have higher luminance Y. Lightness L* is related to luminance Y through the formulaL*=116(Y/Yn)1/3−16; 0.008856<Y/Yn where Yn is the luminance of the white reference. Therefore, in practice, one can work with either quantity. In the 3-dimensional color space illustrated in FIG. 1, lightness L* varies from black to white.
Saturation is the colorfulness of an area judged in proportion to its luminance. Roughly speaking, the more the spectral power distribution (SPD) of a light source is concentrated at one wavelength, the more saturated will be the associated color. A color can be desaturated by adding light that contains power at all wavelengths. In FIG. 1, saturation is the length of the vector extending from the vertical axis.
Hue is the attribute of visual sensation according to which an area appears to be similar to one of the perceived colors, red, yellow, green and blue, or a combination of two of them. Roughly speaking, if the dominant wavelength of the SPD shifts, the hue of the associated color will shift. In FIG. 1, the hue is determined by the angle of the vector.
When capturing images, cameras can have their exposure and color balance settings adjusted. The exposure is adjusted in order to achieve a desired level of luminance. The color balance is adjusted so that the colors in the image are at their desired hues.
In the case of a video camera using a charge coupled device (CCD) as the image sensor, the exposure may be adjusted by adjusting the shutter speed and/or the analog gain. Shutter speed refers to the amount of time which the image sensor is exposed to the image. In a typical CCD application, the shutter speed is controlled electronically by controlling the number of overflow drain pulses (OFD) applied to the sensor to discharge the accumulated charge in the individual CCD pixel sites.
Analog gain refers to the gain of the analog amplifier immediately after the CCD. The signal output from a CCD is an analog signal comprising a reference voltage and a readout voltage for each pixel site. The first stage of processing the video signal output from the sensor is to determine the difference between the reference and readout voltage, and then amplify the difference signal for subsequent processing.
By adjusting the settings for shutter speed and analog gain, the luminance level of pixels in the output signal can be brought in a range optimum for signal processing. The absolute values of the luminance voltage levels at specific colors and intensities are defined by international standard. To ensure accurate signal processing, it is desired to maintain the luminance levels in the midrange of a workable range having an upper limit Max Level (FIG. 6A) and a lower limit Min Level so that noise at low levels and overflow at high levels does not interfere with the processing.
The color balance is adjusted by adjusting the color gains in the digital signal processor (DSP) of a conventional camera. The color gains are adjusted at their desired levels so that the overall image color, when viewed on a vectorscope, is centered around white.
Consider the case where a scene is illuminated with a steady light source and the scene is captured by a conventional color camera with two different shutter speeds. Two slightly different images are obtained. The luminance levels of the two images are different since they were captured with different shutter speeds. The saturation levels of the two images are also different, as saturation depends on lightness, and thus on luminance. However, the hues of the colors in both images should be identical, since the color makeup, the SPD, of the two images is the same, as determined by the steady light source.
Consider now the case where two images of a scene are captured using a different light source for each image, but the same shutter speed and color gain settings in the camera are maintained. The two images are different as the luminance and saturation levels are different. In addition, the hues are different in the two images because the color makeup of the incident light is different in each case, and the same color gain settings have been used for each image.
A standard approach to adjusting the video image to the scene being captured is to adjust the shutter speed of the CCD, and to adjust the analog gain of the CCD output signal, so that the analog output signal is within a desired range of operation. In situations where there is a large contrast between the darkest and brightest areas, the resulting image will either have areas which appear darker in the video image than they actually are, or areas which appear overexposed and brighter than they actually are.
There are prior art video camera systems that try to compensate for non-uniform lighting. Some of these systems use adaptive exposure algorithms that determine two different shutter speeds and/or analog gain settings, for alternating fields of video, targeting bright areas and dark areas, respectively. Typically, these video camera systems use the same color gain settings for each field of video. As a result, the hue values between alternating fields of video are inconsistent.